To fight Prof. Oak, you need to perform the Mew Glitch with a Pokémon that has a Special stat of 226, in my case, Venusaur. P.S. How many times you use Growl on Ditto depends on Oak's team. 0-3 times is a random Poison Type and a random Electric Type. 4 times is Tauros, Exeggutor, Arcanine, Charizard, and Gyarados. 5 times is Tauros, Exeggutor, Arcanine, Venusaur, and Gyarados.6 times is Tauros, Exeggutor, Arcanine, Blastoise, and Gyarados. I went with him using Venusaur since I picked Charmander and Blue picked Squirtle. Skip to 4:48 for the Professor Oak fight.